Ceramic products

ABSTRACT

A method of producing a ceramic product comprising the steps of preparing an aqueous slurry of a silica sol with a refractile material comprising a calcium or zirconium silicate, causing the slurry to gel by physical or chemical means to form a solid structure, and drying said structure to form a porous ceramic product. The product has a high green strength which nevertheless increases on heating, and may be used in building applications.

TECHNICAL FIELD

The present invention relates to ceramic products, and in particular itrelates to ceramic products with high unfired, or "green", strength,especially for use in buildings.

BACKGROUND OF THE INVENTION

It is known to produce fire resistant products for use in buildings andmany of these comprise inorganic material such as asbestos boundtogether into a board or duct. Asbestos is no longer recommended formany applications. These products may take the form of board for use inpartitioning or in cladding steel structures. It is important that thismaterial is itself non-flammable and must exhibit poor thermalconductivity so that the temperature of the flame is dropped across thethickness of the material to an acceptable level. This is particularlyimportant when encasing steel structures since in some cases the steelcan reach a temperature where it will soften and deform.

As ceramic products are fire resistant (although not necessarily havingthe low thermal conductivity of fireboard) they are useful as claddingproducts. However the green strength of most ceramic material limits thesize and complexity of shape that can be made owing to handling problemsbefore firing and/or glazing.

It is an object of the present invention to provide a ceramic materialwhich can be formed into boards or other products which have good fireresistance, and to provide a method of making the same.

SUMMARY OF THE INVENTION

According to the first aspect of the present invention there is provideda method of producing a ceramic product comprising the steps ofpreparing an aqueous slurry of a silica sol with a refractile materialcomprising a calcium silicate and/or zirconium silicate, causing theslurry to gel by physical or chemical means to form a solid structure,and drying said structure to form a porous ceramic product.

The gelation may be induced by means of a chemical gelling agent, but itis currently preferred to use physical means such as pressure mouldingand, in particular, freeze moulding to set the slurry.

Preferably the refractile material, which should be insoluble in water,is a calcium meta-silicate. Preferably, the material includes bothcalcium and zirconium silicates.

The product can be tailored, or explained more fully hereinafter, toproduce a relatively lightweight fireboard or a high green strengthceramic board in large or complex shapes suitable for subsequent firinginto a `tilc` (which term is to include moulded three-dimensional shapesas well as simple square or rectangular sheets).

When the resultant product is a ceramic board, it has been found to haveunexpectedly high green (unfired) strength and, in addition, the surfaceof the product produced using this method is particularly suitable forapplication of ceramic glaze, due to its smoothness, porosity and theabsence of materials in the composition which would damage the glazewhen burned out during firing. Preferred products are large flat orcornered tiles primarily, but not exclusively, for use as internal orexternal decorative architectural wall cladding material.

We believe, although the utility of the invention does not depend on thevariety of this belief, that the high green strength may be due to oneor a combination of the following factors: a) the introduction ofdivalent, as opposed to trivalent (such as Al₂ O₃), cationic refractilematerial; b) the change in pH caused by the addition of acid-neutralzirconium silicates into the alkaline sol; and c) the range ofrefractile particle sizes used, and the total specific surface area ofthese particles in relation to the silica particles, available forbonding. These may all or in part contribute to the formation ofstronger bonds between the silica particles during the gelation processand confer the enhanced green strength properties of the product.

Where a fireboard is required, the product may be of relatively lowdensity, advantageously no more than 850 kg/m³, ideally 500 or even 250kg/m².

The step of drying may be carried by firing the structure or by allowingthe structure to dry under conditions substantially close to ambient.

The slurry is preferably frozen at a temperature in the range -5° C. to-150° C., advantageously in the region of -40° C. to -70° C.

The slurry comprises a colloidal sol of silica advantageously having anaverage particle size less than 30 nanometers.

The step of freezing said slurry may be carried out in a mould with anelement of high thermal conductivity. In this case, the mould may be ofa metal, or heat conductive material, such as aluminium, or a resin,such as an epoxy resin, filled with a metal powder, such as aluminiumpowder.

To produce a fireboard, the slurry may contain void forming material,such as particles of sawdust, polystyrene or the like, and which isburnt out during the step of firing, or a gas-forming agent. Inaddition, thermally resistant materials or strengtheningfibres/materials comprising such as glass fibres, perlite, vermiculite,inorganic lumina, pulverise fuel ash, flake-like materials such as mica,or chopped fibres, e.g. mineral fibres, or such other materials as willgive added strength to the structure, e.g. carbon fibres, may bepresent. The latter may be in the form of individual fibres, plateletsor a mat thereof. As well as improving thermal resistance, the lowerdensity makes the products lighter and easier to handle and install.

According to a second aspect of the present invention there is provideda ceramic product produced in accordance with the above described firstaspect.

The product of the invention can be fired prior to use and will thenassume the strength characteristics typical of ceramics in general. Noloss of strength is observed, indeed strength is increased. This issignificant in that calcium silicates are used in the production offireboards and other products but, although the strength of thesematerials in the green state is relatively high, their strength isreduced or lost upon exposure to very high or sustained temperatures. Itis a unique advantage of the products of the invention that they can beused in the green state, and their strength actually increases ifsubjected to heat, e.g. in the case of a fire. It is preferred to usezirconium silicate (e.g. Zirconsil) in fireboard products as thisenhances strength in conditions of extreme heat for prolonged periods.Since no organic binder is employed there is none to burn out and weakenthe product, and the ability to mould or cast means that more complexshapes than the simple flat boards of hitherto can be made.

The enhanced green strength means that much larger ceramic tiles/boardscan be produced and improves pre-fired handleability to include sawing,routering, sanding and general cutting to required size or shape andease of transportation. The fireboard products have enhanced greenstrength but the ceramic boards have even greater green strength and maybe used unfired or partly fired in situations of heat exposure, wherethe heating will enhance the strength of the product prolonging itslife, but will normally be fired and glazed. In this case the inclusionof void-forming products should be avoided as these will mar the glazeduring firing. Very large glazed tiles can be produced which have manyadvantages over existing wall-cladding systems including the ability toproduce cornered or three-dimensional shapes and to cast-in fixings.

Where firing and/or glazing is carried out the temperature should besufficiently high to at least sinter the product and preferably causecrystallisation of the silica. Temperatures in the range of 700° to1200° C., preferably of 1000° C. or more, may be employed.

Notwithstanding the product's exposure to heat in its untreated greenstate the ceramic board is weather resistant. Thus the increased greenstrength of the product, its stability on firing and ability to glaze,broadens the scope of product applications and will open up new marketsto ceramic products.

The use of calcium or zirconium silicate refractile material fillers(most preferably acicular in nature) in combination with a colloidalsilica sol such as SYTON X30 and gelation by either chemical or physical(pressure) means enables the production of large complexthree-dimensional ceramic bodies with green (unfired) strength of atleast 5 MPa and fired strengths in the range of 10 MPa to 30 MPa asdetermined by a modulus of rupture test.

A preferred refractile material is Wollastonite (calcium metasilicate),which is a mineral whose natural form is acicular (spiny), withlength:diameter ratios from 3:1 to 20:1. The acicular nature of thismaterial is believed to contribute to the green strength of the product.

It is preferred to employ at least 30% silica sol, especially where acalcium silicate is not used. Overall proportions of components may bewithin the following range:

    ______________________________________                                        Silica sol      30-75%                                                        Silicates        8-70%                                                        Other ingredients                                                                              8-40%                                                        ______________________________________                                    

Within this, the proportion of calcium metasilicate should preferablynot exceed 65% (by weight) of the total slurry, and may conveniently bein the range 20-40%. More than one metasilicate may be used, e.g.Wollastonite G and Wollastonite 400, in which case each should be in therange 10-30%. If zirconium silicate, e.g. Zirconsil, is employed itshould preferably be in the range 20-30%.

The ceramic product produced may have a density as high as 2,500 kg/m³but, for a fireboard, preferably has a density below 1500 kg/m³, forexample in the ranges around 250, 500 or 800 kg/m³.

Two or more such products may be joined together at edges thereof byapplying ceramic slip between them, refreezing the combination, and thendrying them to form a unitary product.

According to a third aspect of the present invention there is provided afire-resistant board, tile or casting comprising a ceramic productaccording to the second aspect above.

The casting may be provided with depressions of such depth as toaccommodate ceramic glaze material.

The invention will now be more particularly described by way of exampleand with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of a mould suitable for use in the method of theinvention; and

FIG. 2 is a graph of thermal resistance data of products of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawing, FIG. 1 shows a mould comprising an insulatedlid A and an insulated base B surrounding a lower C and upper D mouldplate separated by spacers E. Slurry F in accordance with the inventionis enclosed between plates C and D. Reservoirs G receive liquid nitrogenin order rapidly to freeze the slurry F.

One preferred step in the method of the present invention is that theceramic slurry is frozen. The freezing temperature may typically beminus 70° C. Since the ceramic slurry contains a freeze-sensitive sol,the volume of the water component of the sol increases on its freezingto ice. This increased volume produces an internal pressure whichdensifies the ceramic particles of the slurry. Subsequently, when thematerial is warmed back up to room temperature, the ice melts andremains as water within the structure but the structure is now solidwith good green strength.

The water is then dried off, to leave a ceramic material which isporous. The amount of porosity in the material is determined by the rateof freezing and the particle size and distribution of particles. Thematerial may then be fired, if so desired.

One advantage of this freezing step is that there is virtually nodimensional change between the wet and dried products. Many ceramicsproduced by conventional routes such as pressing or slip casting shrinkby up to twenty five percent of the original green state ceramicdimensions. Freeze coating eliminates this shrinkage to a very majordegree. The material may also be moulded, possibly continuously toproduce an elongate board.

As stated above, the material is porous and control of the pore size anddistribution is affected by the particle size of the original slurry andby the rate of freezing. The preferred average particle size in the solis less than 30 nanometers. The rate of freezing may be increased byproviding moulds which have high thermal conductivity, such as thosemade of aluminium or an aluminium powder filled epoxy resin. The mouldcan either be immersed in a cryogenic tank or a cryogenic liquid, suchas liquid nitrogen or solid carbon dioxide, from a freezing unit and canbe pumped around channels within the mould. A mould release agent isgenerally used.

Use of the above procedures enables ceramics to be produced between 25%and 85% dense.

The use of particle sizes in the sol of less than 30 nanometers has afurther advantage in that it enables the ceramic to be fired atrelatively low temperatures because of the reactivity of the highsurface area particles. If it is desired further to reduce the densityof the material, e.g. to produce a fireboard, it is possible to addsacrificial materials as described hereinabove.

Fireboards produced in the above manner have extremely low thermalconductivity due to the high porosity but show excellent strength andintegrity at temperatures up to 1200° C. They also have high thermalshock resistance, mechanical integrity and dimensional stability.

Because the freezing step gives a green state product which is itselfstrong or which can be fired without appreciable shrinkage, it ispossible to produce very complex geometries, possibly three-dimensional,of insulation fireboard and ducting using the above method. Also, sincethe system is totally inorganic, and contains no organic binders, thematerial has better temperature stability since there are no bindersystems that can burn out when the material is heated.

The ability to cast complex shapes may be used in the formation ofdecorative tiles. A series of progressively deeper depressions orprofiles may be formed in a surface of the tile, which in use isintended to be outermost. Each depression or profile may be coated orfilled with a glaze so that, when the tile or article is fired, thefinish of the tile shows variation in colour depending on the depth andcolour of the glazes used.

If it is desired to produce a larger product, tiles or other articlesproduced by the method may be joined by applying between them a bondinglayer of ceramic slip, and refreezing the conjoined articles.

It is also possible to incorporate fittings of fixings, such as nuts ortrunking, into the mould so that they become part of the cast article.This enables articles such as tunnel linings, cladding or ceramic glazedbuilding panels to be produced for ease of use at a later date.

The invention is further illustrated in the following non-limitingExamples:

EXAMPLE 1

Composition (wt %):

73% Sodium stabilised silica sol

11% Precipitated silica

8% Zirconium silicate (Zirconsil)

8% Perlite (2JL)

36 ml of polystyrene spheres (3-5 mm) in 100 g of above slurry.

The precipitated silica is first mixed thoroughly into the colloidalsilica until it is completely dispersed. Next, zirconsil powder isdispersed by continuous stirring until a uniform suspension is obtained.Finally, the perlite and polystyrene are spheres are added and mixed in.

The slurry is poured into a mould to produce the required shape, themould sealed and then frozen with liquid nitrogen to -70° C. Uponwarming back up to room temperature a solid structure has formed. Whendry, the product is subjected to a low temperature `firing` step at 800°C. for one hour which produces a porous ceramic having a density of 400kg/cu.m. The temperature used in below that at which sintering takesplace and does not contribute to the strength of the board. It isemployed to burn out the heat labile ingredients in order to producevoids in the fireboard.

EXAMPLE 2

Composition (wt %):

71% Sodium stabilised colloidal silica sol

14% Zirconsil powder

7% Quartz sand

8% Perlite (2JL)

The Zirconsil powder is first dispersed in the sol and then the otheringredients mixed in.

A mat of inorganic fibres is introduced into the mould and the aboveslurry poured in. A second mat of inorganic fibre is then introduced ontop of the slurry. A board was then freeze cast as described in Example1, except that it was dried at 200° C. and fired at 850° C. for onehour. The board so obtained had a density of 560 kg/cu.m and appeared tohave greater mechanical strength than that of Example 1.

The board was strong enough to be handled and was tested for thermalinsulation by exposing one surface of the board to the face of an ovenheated to 1000° C. and recording the temperature on the other surfaceover time. The results appear in FIG. 2. After the test the boardretained its integrity and showed no visible dimensional change afterexposure to 1000° C. for eight hours.

EXAMPLE 3

Composition (wt %):

56.3% Colloidal silica sol (sodium stabilised)

3.8% Precipitated silica

7.0% Quartz sand

36.3% Zirconsil powder (ZrO₂.SiO₂)

6.1% Perlite (2JL)

0.5% Chopped glass fibre

The precipitated silica was first dispersed in the colloidal silica solfollowed by the other products.

The board was then freeze cast as described in Example 1 except that theboard was dried at room temperature for several hours before furtherdrying at 100° C. and firing at 800° C. for one hour. The board had afinal density of 800 kg/cu.m. The thermal insulation test results areshown in FIG. 2.

EXAMPLE 4

Composition (wt %):

53.3% Syton X30 (silica sol)

13.3% Wollastonite (NYAD G)

13.3% Wollastonite (NYAD 400)

13.3% Zirconsil

6.6% Vermiculite (fine)

The designations NYAD G or 400 refer to the supplier's (Cooksons, Stokeon Trent) designation of the grade.

The Wollastonite C, followed by the Wollastonite 400, the zirconsil andfinally the vermiculite were added to the sol in that order and mixedin. The board was freeze cast as described in Example 1 except that itwas dried at room temperature and then at 100° C. overnight. No `firing`was carried out. The board had a final density of 920 kg/cu.m and wasthermally tested as in Example 3. Test results are shown in FIG. 2 andafter 1 hour at 1000° C. no damage was visible to either surface of theboard. The board was strength tested using a standard modulus of rupturetest and the results are shown in Table 1 below together withcomparative tests on proprietary fireboards Supalux and Promatect.

EXAMPLE 5

Composition (wt %):

    ______________________________________                                        Silica Sol (SYTON X30)     33.5%                                              Calcium Metasilicate (Wollastonite NYAD G)                                                               16.5%                                              Calcium Metasilicate (Wollastonite NYAD 400)                                                             25%                                                Zirconium Silicate (Zirconsil)                                                                           25%                                                ______________________________________                                    

The Syton X30 was weighed and placed in a mixing container. The otheringredients were individually stirred into the Syton in the followingorder: Wollastonite G, Wollastonite 400 and Zirconsil. An industrialwhisk-type mixer was used to combine the ingredients.

Once combined the slurry is stable at room temperature for 24 hours,however some sedimentation does occur requiring the slurry to berestirred before use.

An aluminium mould was constructed such that the internal dimensionswere 1200×1300×9 mm (see FIG. 1).

The slurry was poured into the mould to slight excess volume such thatwhen the sixth side was bolted on, the excess slurry was separated out.This ensured that no air pockets were created.

The mould was then subject to cooling using liquid nitrogen which waspoured into a bath containing the mould. The freezing process wasallowed to continue for a minimum of 6 minutes and temperaturemaintained at -30°--40° C. for a minimum further 6 minutes.

The ceramic board was removed from the mould and dried at a temperatureof 150° C. for 2.5 hours. This produces a board with a high greenstrength (see Table 1).

Boards were either fired whole at 1190° C. or cut into smaller piecesand fired with or without glaze, and then strength tested on a universaltesting machine (100 centres and 1.5 mm/min) and the MPa required tofracture the tile recorded, as above. The results are given in Table 1.

EXAMPLE 6

37.7% Syton X30 (silica sol)

18.6% Wollastonite NYAD G

43.7% Wollastonite NYAD 400

The wollastonite G and 400 were added to the sol in that order asdescribed in Example 4. The slurry was poured into a mould and freezecast as in example 5. Samples were tested in the green state and afterfiring both with and without glaze. The results are in Table 1.

                  TABLE 1                                                         ______________________________________                                        SAMPLE               STRENGTH MPa                                             ______________________________________                                        Supalux (Cape)       Av. 6.325 N = 4                                          Promatec L (Eternit) Av. 3.1 N = 4                                            Example 4            Av. 2.24 N = 4                                           Example 5 (Green State)                                                                            Av. 8.5 N = 2                                            Example 5 (Fired)    Av. 12.6 N = 3                                           Example 5 (Glazed)   Av. 16.8 N = 3                                           Example 6 (green state)                                                                            Av. 5.7 N = 2                                            Example 6 (Fired)    Av. 11.33 N = 2                                          Example 6 (Glazed)   Av. 11.5 N = 2                                           ______________________________________                                    

We claim:
 1. A method of producing a ceramic product comprising thesteps of preparing an aqueous slurry of silica sol with a refractorymaterial comprising a calcium silicate in the substantial absence oftrivalent cationic refractory material, freezing the closed mold tocause the slurry to gel to form a solid monolithic structure, and dryingsaid structure to form a solid ceramic product.
 2. A method as claimedin claim 1 wherein the refractory material is a calcium meta-silicate.3. A method as claimed in claim 2 wherein a zirconium silicate is alsopresent.
 4. A method as claimed in claim 1 wherein the step of drying iscarried out by firing the structure or by allowing the structure to dryunder conditions substantially close to ambient.
 5. A method as claimedin claim 1 wherein the slurry is frozen at a temperature in the range of-5° C. to -150° C.
 6. A method as claimed in claim 1 wherein the slurrycomprises a colloidal sol of silica having an average particle size lessthan 30 nanometers.
 7. A method as claimed in claim 1 wherein the slurrycontains void forming material selected from the group consisting ofparticles of sawdust and polystyrene and which is burnt out during thestep of firing.
 8. A method as claimed in claim 1 further includingthermally insulating and strengthening the final product by adding tothe glass substances selected from the group consisting of glass fibers,perlite, vermiculite, inorganic lamina, pulverised fuel ash, flake-likematerials and carbon fibres.
 9. A method as claimed in claim 1 whereinthe refractory material is acicular.
 10. A method as claimed in claim 1in which the product in its green state has a strength at least 5 MPaand a fired strength in the range of 10 MPa to 30 MPa as determined by amodulus of rupture test.
 11. A method as claimed in claim 1 wherein theproportion of calcium silicate should not exceed 65% (by weight) of thetotal slurry.
 12. A method of producing a ceramic board comprising thesteps of preparing an aqueous slurry of a silica sol with refractorymaterial comprising calcium metasilicate, freezing the slurry in aclosed mold to cause the slurry to gel to form a solid monolithicstructure, drying the board and glazing or firing it to produce thefinished product.
 13. A ceramic product produced by a method as claimedin any of claims 1, or
 12. 14. A fire-resistant board, tile or castingcomprising a ceramic product according to claim
 13. 15. A board asclaimed in claim 14 having depressions or profiles of such depth as toaccommodate a ceramic glaze material.